Optical system with compensation lens

ABSTRACT

An optical system used in a charged particle beam inspection system. The optical system includes one or more optical lenses, and a compensation lens configured to compensate a drift of a focal length of a combination of the one or more optical lenses from a first medium to a second medium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. application 62/564,966 which was filed on Sep. 28, 2017, and which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

The present disclosure generally relates to an optical system in a charged particle beam inspection system and, more specifically, to an optical system with a compensation lens in a charged particle beam inspection system.

BACKGROUND

A charged particle beam based microscope, such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM), capable of providing image resolution down to less than a nanometer, serves as a practicable tool for inspecting samples having a feature size that is sub-100 nanometers. During operation of the charged particle beam based microscope, a part of the microscope is positioned in a vacuum chamber.

An optical system is usually included in the charged particle beam based microscope to allow visual inspection of a sample, to detect sample height, or to illuminate the sample. During operation of the charged particle beam based microscope, one or more lenses of the optical system are position in the vacuum chamber along with the part of the charged particle beam based microscope. As the medium of the optical system changes, the focal length of the one or more lenses may change, affecting the precision of the optical system.

SUMMARY

According to some embodiments of the disclosure, an optical system used in a charged particle beam inspection system is provided. The optical system includes one or more optical lenses, and a compensation lens configured to compensate a drift of a focal length of a combination of the one or more optical lenses from a first medium to a second medium.

According to some embodiments of the disclosure, a method for operating an optical system used in a charged particle beam inspection system is provided. The optical system including one or more optical lenses. The method includes: providing a compensation lens in the optical system, the compensation lens being configured to compensate a drift of a focal length of a combination of the one or more lenses from a first medium to a second medium; assembling and calibrating the optical system in the first medium; removing the compensation lens; and placing the one or more optical lenses in the second medium.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments.

FIG. 1 is a schematic diagram of an exemplary charged particle beam inspection system with an optical position detection system, consistent with some disclosed embodiments.

FIG. 2 is a schematic diagram of an exemplary charged particle beam inspection system with an optical imaging system, consistent with some disclosed embodiments.

FIG. 3 is a schematic diagram of an exemplary charged particle beam inspection system with an optical illumination system, consistent with some disclosed embodiments.

FIG. 4 schematically illustrates an exemplary method for configuring a compensation lens for use in an optical system in a charged particle beam inspection system, consistent with some disclosed embodiments.

FIG. 5 is a flow chart of an exemplary process of operating an optical system in a charged particle beam inspection system, consistent with some embodiments of the disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to the example embodiments, which are illustrated in the accompanying drawings. Although the following embodiments are described in the context of utilizing electron beams, the disclosure is not so limited. Other types of charged particle beams can be similarly applied.

The disclosed embodiments provide an optical system used in a charged particle beam inspection system. The optical system includes one or more optical lenses to be disposed under vacuum atmosphere, and a compensation lens configured to compensate a drift of a focal length of a combination of the one or more optical lenses from atmospheric conditions to vacuum conditions. The optical system and the charged particle beam inspection system are assembled and calibrated under atmospheric conditions with the compensation lens. When a part of charged particle beam inspection system including the one or more optical lenses of the optical system is placed under vacuum conditions, a drift of a focal length of a combination of the one or more optical lenses has been compensated by the compensation lens.

FIG. 1 is a schematic diagram of a charged particle beam inspection system 100 with an optical position detection system 120, consistent with some disclosed embodiments. Charged particle beam inspection system 100 includes a charged particle beam column 110 for irradiating a charged particle beam (e.g., electron beam) on an inspected sample 102, and optical position detection system 120 for measuring and calibrating a height of sample 102 (e.g., a position of sample 102 along a Z-direction illustrated in FIG. 1). Optical position detection system 120 includes a projection module 130, a receiving module 140, and a detection module 150. Projection module 130 includes an illumination module 132, a grating mask 134, a projection lens 136, and a mirror 138. Projection module 130 is configured to project a light beam to sample 102. Receiving module 140 includes a mirror 142 and a receiving lens 144. Receiving module 140 is configured to receive a light beam reflected from sample 102. Detection module 150 includes a vacuum window 152, a compensation lens 154, and a detector 156. Detection module 150 is configured to detect a position of the sample based on the light beam reflected from sample 102. Charged particle beam column 110, projection module 130, receiving module 140, and sample 102 are disposed in an inspection chamber 160.

Grating mask 134 includes a substrate and a grating pattern formed on the substrate. The grating pattern includes a plurality of grating members arranged periodically along a direction parallel to the substrate. A light transmittance of the grating members is lower than a light transmittance of the substrate.

When illumination module 132 emits a light beam downward along a Z-direction perpendicular to a surface of sample 102, the light beam transmits through grating mask 134 having the grating pattern. Consequently, the light beam carries the grating pattern of grating mask 134. The light beam is conjugated by projection lens 136, and reflected by mirror 138 towards sample 102. At a surface of sample 102, the light beam is reflected by sample 102 to form a grating image. The reflected light beam transmits towards receiving module 140. In receiving module 140, the light beam is reflected by mirror 142 upward along the Z-direction towards receiving lens 144, and is conjugated by receiving lens 144. In detection module 150, the light beam passes through vacuum window 152 and compensation lens 154, and reaches detector 156, which detects the grating image. Detector 156 compares the detected grating image with the grating pattern, and determines the position of sample 102 along the Z-direction based on the comparison.

As described above, optical position detection system 120 is used for measuring and calibrating the height of sample 102, so as to accurately control the height of sample 102. The accuracy of the height of sample is essential for controlling focusing condition of charged particle beam column 110, and thus affecting the inspection accuracy of charged particle beam inspection system 100.

Charged particle beam inspection system 100 is assembled and calibrated under atmospheric conditions (e.g., in the air). After charged particle beam inspection system 100 is assembled and calibrated, inspection chamber 160 is evacuated to generate vacuum, such that charged particle beam column 110, projection module 130, receiving module 140, and sample 102 are placed under vacuum conditions. Due to the difference between the refractive indices of air and vacuum, the focal length of projection lens 136 of projection module 130 and the focal length of receiving lens 144 of receiving module 140 may drift. The drift in the focal lengths may affect the accuracy of the detected height of sample 102.

In order to solve this problem, in the disclosed embodiments, compensation lens 154 is designed to compensate a drift of a focal length of a combined lens group consisting of projection lens 136 and receiving lens 144 from the atmospheric conditions to the vacuum conditions. In other words, when projection lens 136 and receiving lens 144 are placed under atmospheric conditions, compensation lens 154 can emulate an effect where projection lens 136 and receiving lens 144 are placed under vacuum conditions. A method for configuring compensation lens 154 will be described with references to FIG. 4.

When charged particle beam inspection system 100 including optical position detection system 120 is assembled and calibrated under atmospheric conditions, compensation lens 154 is placed in detection module 150 to emulate an effect when projection lens 136 and receiving lens 144 are placed under vacuum conditions. Therefore, when charged particle beam inspection system 100 is assembled and calibrated with compensation lens 154 under atmospheric conditions, charged particle beam inspection system 100 is in an effect of being calibrated under vacuum conditions. After charged particle beam inspection system 100 is assembled and calibrated under the atmospheric conditions, compensation lens 154 is removed from detection module 150, and inspection chamber 160 is evacuated to place charged particle beam column 110, projection module 130, receiving module 140, and sample 102 under vacuum conditions. Because charged particle beam inspection system 100 has been calibrated with compensation lens 154, a drift of a focal length of the combined lens group consisting of projection lens 136 and receiving lens 144 has been corrected by compensation lens 154. As a result, the height of sample 102 can be accurately detected by optical position detection system 120.

FIG. 2 is a schematic diagram of a charged particle beam inspection system 200 with an optical imaging system 220, consistent with some disclosed embodiments. Charged particle beam inspection system 200 includes a charged particle beam column 210 for irradiating a charged particle beam (e.g., electron beam) on an inspected sample 202, and optical imaging system 220 for obtaining an image of sample 202. Optical imaging system 220 includes an illumination module 230, a detection module 240, and an objective lens 260. Illumination module 230 is configured to project a light beam to sample 202. Objective lens 260 is configured to conjugate a light beam reflected from sample 202. Detection module 240 includes a detector 242, a compensation lens 244, a tube lens 246, a beam splitter 248, and a vacuum window 250. Detection module 240 is configured to detect an image of sample 202 based on the light beam reflected from sample 202. Charged particle beam column 210, objective lens 260, and sample 202 are disposed in an inspection chamber 270.

When illumination module 230 emits a light beam along an X-direction parallel to a surface of sample 202, part of the light beam is reflected by beam splitter 248 downwards along a Z-direction perpendicular to the surface of sample 202. The light beam is conjugated by objective lens 260 onto a surface of sample 202. At the sample surface, the light beam is reflected by sample 202 to form an image of sample 202. The reflected light beam is conjugated by objective lens 260 towards detection module 240. In detection module 240, the light beam passes through vacuum window 250 and beam splitter 248. Tube lens 246 magnifies the image of sample 202 formed by the light beam. The light beam then passes through compensation lens 244, and reaches detector 242, which detects the image of sample 202.

Charged particle beam inspection system 200 is assembled and calibrated under atmospheric conditions (e.g., in the air). When charged particle beam inspection system 200 is calibrated under the atmospheric conditions, the positions of objective lens 260 and sample 202 are adjusted to achieve an optimum imaging condition (e.g., the light beam reflected by sample 202 is focused on detector 242). After charged particle beam inspection system 200 is assembled and calibrated, inspection chamber 270 is evacuated to generate vacuum, such that charged particle beam column 210, objective lens 260, and sample 202 are placed under a vacuum conditions. Due to the difference between the refractive indices of air and vacuum, the focal length of objective lens 260 may drift. As a result, the positions of objective lens 260 and sample 202 that were previously adjusted in the atmospheric conditions may no longer achieve the optimum imaging condition.

In order to solve this problem, in the disclosed embodiments, compensation lens 244 is designed to compensate a drift of a focal length of objective lens 260 from the atmospheric conditions to the vacuum conditions. In other words, when objective lens 260 is placed under atmospheric conditions, compensation lens 244 can emulate an effect where objective lens 260 is placed under vacuum conditions. When charged particle beam inspection system 200 is assembled and calibrated under atmospheric conditions, compensation lens 244 is placed in detection module 240 to emulate an effect where objective lens 260 is placed under vacuum conditions. Therefore, when charged particle beam inspection system 200 is assembled and calibrated with compensation lens 244 under atmospheric conditions, charged particle beam inspection system 200 is in an effect of being calibrated under vacuum conditions. After charged particle beam inspection system 200 is assembled and calibrated under atmospheric conditions, compensation lens 244 is removed from detection module 240, and inspection chamber 270 is evacuated to place charged particle beam column 210, objective lens 260, and sample 202 under vacuum conditions. Because charged particle beam inspection system 200 has been calibrated with compensation lens 244, a drift of a focal length of objective lens 260 has been corrected by compensation lens 244. As a result, optimum imaging condition can be achieved.

FIG. 3 is a schematic diagram of a charged particle beam inspection system 300 with an optical illumination system 320, consistent with some disclosed embodiments. Charged particle beam inspection system 300 includes a charged particle beam column 310 for irradiating a charged particle beam (e.g., electron beam) on an inspected sample 302, and optical illumination system 320 for obtaining an image of sample 302. Optical illumination system 320 includes an illumination module 330, a detection module 340, an objective lens 360, and a mirror 370. Illumination module 330 is configured to project a light beam to a portion of sample 302 under charged particle beam column 310. Objective lens 360 is configured to conjugate a light beam scattered from sample 302. Detection module 340 includes a detector 342, a tube lens 344, a beam splitter 346, a compensation lens 348, and a vacuum window 350. Detection module 340 is configured to detect an image of the portion of sample 302 based on the light beam scattered from sample 302. Charged particle beam column 310, objective lens 360, mirror 370, and sample 302 are disposed in an inspection chamber 380.

When illumination module 330 emits a light beam along an X-direction parallel to a surface of sample 302, part of the light beam is reflected by beam splitter 346 downwards along a Z-direction perpendicular to the surface of sample 302. The light beam passes through compensation lens 348 and vacuum window 350, and is conjugated by objective lens 360. The conjugated light beam is reflected by mirror 370 onto a portion of sample 302 directly below charged particle beam column 310. The light beam is reflected by sample 302 to form an image of sample 302. The reflected light beam is further reflected by mirror 370 upwards along the Z-direction. The reflected light beam is conjugated by objective lens 360 towards detection module 340. In detection module 340, the light beam passes through vacuum window 350, compensation lens 348, and beam splitter 346. Tube lens 344 magnifies the image of sample 302 formed by the light beam. The light beam then reaches detector 342, which detects the image of sample 302.

Charged particle beam inspection system 300 is assembled and calibrated under atmospheric conditions (e.g., in the air). When charged particle beam inspection system 300 is calibrated under the atmospheric conditions, the positions of objective lens 360, mirror 370, and sample 302 are adjusted to achieve an optimum imaging condition (e.g., the light beam reflected by sample 302 is focused on detector 342). After charged particle beam inspection system 300 is assembled and calibrated, inspection chamber 380 is evacuated to generate a vacuum, such that charged particle beam column 310, objective lens 360, mirror 370, and sample 302 are placed under a vacuum conditions. Due to the difference between the refractive indices of air and vacuum, the focal length of objective lens 360 may drift. As a result, the positions of objective lens 360, mirror 370, and sample 302 that were previously adjusted in the atmospheric conditions may no longer achieve the optimum imaging condition.

In order to solve this problem, in the disclosed embodiments, compensation lens 348 is configured to compensate a drift of a focal length of objective lens 360 from the atmospheric conditions to the vacuum conditions. In other words, when objective lens 360 is placed under atmospheric conditions, compensation lens 348 can emulate an effect where objective lens 360 is placed under vacuum conditions. When charged particle beam inspection system 300 is assembled and calibrated under the atmospheric conditions, compensation lens 348 is placed in detection module 340 to emulate an effect where objective lens 360 is placed under vacuum conditions. Therefore, when charged particle beam inspection system 300 is assembled and calibrated with compensation lens 348 under atmospheric conditions, charged particle beam inspection system 300 is in an effect of being calibrated under vacuum conditions. After charged particle beam inspection system 300 is assembled and calibrated under the atmospheric conditions, compensation lens 348 is removed from detection module 340, and inspection chamber 380 is evacuated to place charged particle beam column 310, objective lens 360, mirror 370, and sample 302 under vacuum conditions. Because charged particle beam inspection system 300 has been calibrated with compensation lens 348, a drift of a focal length of objective lens 360 has been corrected by compensation lens 348. As a result, optimum imaging condition can be achieved.

FIG. 4 schematically illustrates a method for configuring a compensation lens for use in an optical system in a charged particle beam inspection system, consistent with some disclosed embodiments. The method illustrated in FIG. 4 can be applied for configuring, e.g., compensation lens 154 in optical position detection system 120 of FIG. 1.

As illustrated in FIG. 4, an optical system 400 includes an illumination source 410, a first lens 420 (e.g., projection lens 136), a sample 430 (e.g., sample 102), a second lens 440 (e.g., receiving lens 144), a compensation lens 450 (e.g., compensation lens 154), and a detector 460 (e.g., detector 156). First lens 420 and second lens 440 constitute a combined lens group that has a focal plane. Ideally, detector 460 is disposed at the focal plane of the combined lens group. Illumination source 410, first lens 420, sample 430, and second lens 440 are disposed in an inspection chamber 470 which can be evacuated to create a vacuum conditions. Compensation lens 450 and detector 460 are disposed outside of inspection chamber 470.

When inspection chamber 470 is evacuated to switch the environment of illumination source 410, first lens 420, sample 430, and second lens 440 from air to vacuum, the focal length of the combined lens group consisting of first lens 420 and second lens 440 may drift (e.g., the position of the focal plane of the combined length group may shift in a X-direction) due to a difference between the refractive index of air and the refractive index of vacuum. The drift of the focal length can be represented by an image distance change dV, where V is an image distance of the combined lens group. The image distance change dV can be represented by dV=dV₁+dV_(M), where dV₁ is a drift of the focal length purely resulted from the change of the refractive index from vacuum to air, and dV_(M) is a drift of the focal length purely resulted from axial magnification, and dV_(M)=M²*dU, where M represents a lateral magnification, and dδ=dU≠0 is the objective side principle plane drift. dU represents a drift of an objective side principle plane of the combined lens group resulted from the change of the refractive index from vacuum to air. As illustrated in FIG. 4, P represents an objective side principle plane of the combined lens group under the vacuum conditions, P_(a) represents an objective side principle plane of the combined lens group under the atmospheric conditions, P′ represents an image side principle plane of the combined lens group under the vacuum conditions, P_(a)′ represents an image side principle plane of the combined lens group under the atmospheric conditions, U represents an object distance of the combined lens group, dU represents an object distance change (i.e., the distance between P and P_(a)) resulted from the refractive index change, and δ represents the distance between first lens 420 and P.

Assuming that first lens 420 has an optical power Φ₁ under atmospheric conditions, and second lens 440 has an optical power Φ₂ under atmospheric conditions. Each one of the optical power Φ₁ and Φ₂ can be represented by:

$\begin{matrix} {\Phi_{j} = {\frac{n_{gj} - n}{n_{gj} - 1}\Phi_{j\; 0}}} & (1) \end{matrix}$

where j=1 or 2, n_(gj) is the glass refractive index of first lens 420 or second lens 440, n is the refractive index of air, and 1 is the refractive index of vacuum, and Φ_(j0) is the optical power of first lens 420 or second lens 440 under vacuum conditions.

A change dΦ₁ of the optical power Φ₁ of first lens 420 or a change dΦ₂ of the optical power Φ₂ of second lens 440 induced by the refractive index change dn from vacuum to air can be represented by:

$\begin{matrix} {{d\; \Phi_{j}} = {{- \frac{\Phi_{j\; 0}}{n_{gj} - 1}}{dn}}} & (2) \end{matrix}$

where j=1 or 2.

Assuming that a combined lens group consisting of first lens 420 and second lens 440 has an optical power Φ under atmospheric conditions, the optical power Φ can be represented by:

$\begin{matrix} {\Phi = {\Phi_{1} + \Phi_{2} - {\frac{t}{n}\Phi_{1}\Phi_{2}}}} & (3) \end{matrix}$

where t is the distance between first lens 420 and second lens 440.

A change dΦ of the optical power Φ of the combined lens group induced by the refractive index change dn from vacuum to air can be represented by:

dΦ=dΦ ₁ +dΦ ₂ +tΦ ₁Φ₂ dn−tΦ ₁ dΦ ₂ −tΦ ₂ dΦ ₁=(1−tΦ ₂₀)dΦ ₁+(1−tΦ ₁₀)dΦ ₂ +tΦ ₁₀Φ₂₀ dn   (4)

A relationship between the object distance U of the combined lens group and the image distance of the V combined lens group can be represented by:

$\begin{matrix} {{\frac{1}{U} + \frac{1}{V}} = \frac{\Phi}{n}} & (5) \end{matrix}$

From Equation (5), the image distance change dV induced by the refractive index change dn from vacuum to air can be represented by:

$\begin{matrix} {{dV} = {{{dV}_{1} + {dV}_{M}} = {{V_{0}^{2}\left( {{\Phi \; {dn}} - {d\; \Phi}} \right)} - {\left( \frac{V}{U} \right)^{2}{dU}}}}} & (6) \end{matrix}$

dV₁ and dV_(M) can be respectively represented by:

dV ₁ =V ₀ ²[(Φ₁+Φ₂−2tΦ ₁Φ₂)dn+(tΦ ₂−1)dΦ ₁+(tΦ ₁−1)dΦ ₂]  (7)

dV _(M) =−M ² dU   (8)

Therefore, the image distance change dV induced by the refractive index change dn from vacuum to air can be determined based on the optical power Φ₁ of first lens 420, the optical power Φ₂ of second lens 440, the distance t between first lens 420 and second lens 440, and the refractive indices of vacuum and air, based on Equations (1)-(8).

Based on optical theory, the position of the objective side principle plane of the combined lens group can be represented by:

$\begin{matrix} {\delta = {\frac{t}{n}\frac{\Phi_{2}}{\Phi}}} & (9) \end{matrix}$

From Equation (9), the object distance change dU can be represented by:

$\begin{matrix} {{dU} = {{d\; \delta} = {{{- \frac{t\; \Phi_{20}}{\Phi_{0}}}{dn}} + {t\left( {{\frac{1}{\Phi_{0}}d\; \Phi_{2}} - {\frac{\Phi_{20}}{\Phi_{0}^{2}}d\; \Phi}} \right)}}}} & (10) \end{matrix}$

where Φ₂₀ is the optical power of second lens 440 in vacuum, and Φ₀ is the optical power of the combined lens group in vacuum.

As illustrated in FIG. 4, compensation lens 450 is disposed outside of inspection chamber 470, between second lens 440 and detector 460 of the combined lens group. An object distance U_(c) of compensation lens 450 can be represented by:

U _(c) =V _(c) −dV   (11)

where V_(c) represents an image distance of compensation lens 450. Using Equation (11), the image distance V_(c) of compensation lens 450 can be determined based on the object distance U_(c) and the image distance change dV induced by the refractive index change dn from vacuum to air by using Equation (11).

An optical power Φ_(c) of compensation lens 450 can be represented by:

$\begin{matrix} {\Phi_{c} = {\frac{1}{U_{c}} + \frac{1}{V_{c}}}} & (12) \end{matrix}$

Therefore, the optical power Φ_(c) of compensation lens 450 can be determined based on the object distance U_(c) of compensation lens 450, and the image distance V_(c) of compensation lens 450 by using Equation (12). Once the optical power Φ_(c) of compensation lens 450 is determined, compensation lens 450 can be formed and used in the optical system to compensate the change dΦ of the optical power Φ of the combined lens group.

As discussed above, the method described above with respect to FIG. 4 can be applied for configuring compensation lens 154 in optical position detection system 120 of FIG. 1. Compensation lens 244 in optical imaging system 220 of FIG. 2, or compensation lens 348 in optical illumination system 320 of FIG. 3 can be configured using a method similar to the method described above with respect to FIG. 4.

FIG. 5 is a flow chart of a process 500 of operating an optical system in a charged particle beam inspection system, consistent with some embodiments of the disclosure. The charged particle beam inspection system can be one of charged particle beam inspection systems 100, 200, and 300 respectively illustrated in FIGS. 1-3.

As illustrated in FIG. 5, first, the optical system is assembled under atmospheric conditions (step 510). The optical system can be optical position detection system 120 illustrated in FIG. 1, optical imaging system 220 of FIG. 2, or optical illumination system 320. The optical system includes one or more lenses to be disposed under vacuum conditions. A compensation lens is provided in the optical system (step 512). The compensation lens is configured to compensate a drift of a focal length of a combination of the one or more lenses from atmospheric conditions to vacuum conditions. The optical system provided with the compensation lens is calibrated under atmospheric conditions (step 514). After the optical system with the compensation lens is calibrated, the compensation lens is removed from the optical system (step 516). Next, at least a part of the optical system is placed under vacuum conditions (step 518). Once the optical system is placed under vacuum conditions, the optical system can be used to detect sample height, inspect sample surface, or illuminate sample.

According to the above disclosed embodiments, an optical system in a charged particle beam inspection system is assembled and calibrated under atmospheric conditions with a removable compensation lens configured to compensate a drift of a focal length of a combination of one or more lenses in the optical system from atmospheric conditions to vacuum conditions. When a part of the optical system is positioned in a vacuum chamber during the operation of the charged particle beam inspection system, the drift of the focal length of the one or more lenses in the optical system has been corrected by the compensation lens when the optical system is assembled and calibrated. As a result, the precision of the optical system is improved without changing the original design of the optical system.

In the embodiments described in FIGS. 1-3, charged particle beam inspection systems 100, 200, and 300 each includes a singular charged particle beam column 110, 210, and 310, and each one of charged particle beam columns 110, 210, and 310 irradiates a singular charged particle beam. However, the present disclosure is not limited thereto. That is, at least one of a charged particle beam columns 110, 210, and 310 can irradiate multiple charged particle beams. In addition, at least one of charged particle beam inspection systems 100, 200, and 300 can include multiple charged particle beam columns.

The embodiments may further be described using the following clauses:

-   1. An optical system used in a charged particle beam inspection     system, comprising:     -   one or more optical lenses; and     -   a compensation lens configured to compensate a drift of a focal         length of a combination of the one or more optical lenses from a         first medium to a second medium. -   2. The optical system of clause 1, wherein the optical system is a     position detection system comprising:     -   a projection module including a projection lens and configured         to project a first light beam to a sample;     -   a receiving module including a receiving lens and configured to         receive a second light beam reflected from the sample; and     -   a detection module configured to detect a position of the sample         based on the second light beam. -   3. The optical system of clause 2, wherein the one or more optical     lenses include the projection lens included in the projection module     and the receiving lens included in the receiving module. -   4. The optical system of any one of clauses 2 and 3, wherein the     detection module is disposed in the first medium, and the projection     module and the receiving module are disposed in the second medium. -   5. The optical system of clause 1, wherein the optical system is an     imaging system comprising:     -   an illumination module configured to project a first light beam         to a sample;     -   an objective lens configured to conjugate a second light beam         reflected from the sample; and     -   a detection module configured to detect an image of the sample         based on the second light beam. -   6. The optical system of clause 5, wherein the one or more optical     lenses include the objective lenses. -   7. The optical system of any one of clauses 5 and 6, wherein the     detection module is disposed in the first medium, and the objective     lens is disposed in the second medium. -   8. The optical system of clause 1, wherein the optical system is an     illumination system comprising:     -   an illumination module configured to project a first light beam         to a portion of a sample under a charged particle beam;     -   an objective lens configured to conjugate a second light beam         reflected from the sample; and     -   a detection module configured to detect an image of the portion         of the sample based on the second light beam. -   9. The optical system of clause 8, wherein the one or more optical     lenses include the objective lenses. -   10. The optical system of any one of clauses 8 and 9, wherein the     detection module is disposed in the first medium, and the objective     lens is disposed in the second medium. -   11. The optical system of any one of clauses 1 through 10, wherein     the first medium is air, and the second medium is vacuum. -   12. The optical system of any one of clauses 1 through 11, wherein     the charged particle beam inspection system is an electron beam     inspection system. -   13. The optical system of any one of clauses 1 through 12, wherein a     focal length of the compensation lens is configured based on a focal     length of each one of the one or more lenses, a position of each one     of the one or more lenses, and refractive indices of the first     medium and the second medium. -   14. A method for operating an optical system used in a charged     particle beam inspection system, the optical system including one or     more optical lenses, the method comprising:     -   providing a compensation lens in the optical system, the         compensation lens being configured to compensate a drift of a         focal length of a combination of the one or more lenses from a         first medium to a second medium;     -   assembling and calibrating the optical system in the first         medium;     -   removing the compensation lens; and     -   placing the one or more optical lenses in the second medium. -   15. The method of clause 14, wherein the optical system is a     position detection system, and the method further includes:     -   projecting, by a projection module including a projection lens,         a first light beam to a sample;     -   receiving, by a receiving module including a receiving lens, a         second light beam reflected from the sample; and     -   detecting, by a detection module, a position of the sample based         on the second light beam -   16. The method of clause 15, wherein the one or more optical lenses     include the projection lens included in the projection module and     the receiving lens included in the receiving module. -   17. The method of any one of clauses 15 and 16, wherein the     detection module is disposed in the first medium, and the projection     module and the receiving module are disposed in the second medium. -   18. The method of clause 14, wherein the optical system is an     imaging system, and the method further comprises:     -   projecting, by an illumination module, a first light beam to a         sample;     -   conjugating, by an objective lens, a second light beam reflected         from the sample; and     -   detecting, by a detection module, an image of the sample based         on the second light beam. -   19. The method of clause 18, wherein the one or more optical lenses     include the objective lenses. -   20. The method of any one of clauses 18 and 19, wherein the     detection module is disposed in the first medium, and the objective     lens is disposed in the second medium. -   21. The method of clause 14, wherein the optical system is an     illumination system, and the method further comprises:     -   projecting, by an illumination module, a first light beam to a         portion of a sample under a charged particle beam;     -   conjugating, by an objective lens, a second light beam reflected         from the sample; and     -   detecting, by a detection module, an image of the portion of the         sample based on the second light beam. -   22. The method of clause 21, wherein the one or more optical lenses     include the objective lenses. -   23. The method of any one of clauses 21 and 22, wherein the     detection module is disposed in the first medium, and the objective     lens is disposed in the second medium. -   24. The method of any one of clauses 14 through 23, wherein the     first medium is air, and the second medium is vacuum. -   25. The method of any one of clauses 14 through 24, wherein the     charged particle beam inspection system is an electron beam     inspection system. -   26. The method of any one of clauses 14 through 25, further     comprising:     -   determining a focal length of the compensation lens based on a         focal length of each one of the one or more lenses, a position         of each one of the one or more lenses, and refractive indices of         the first medium and the second medium.

While the present invention has been described in connection with various embodiments, other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. 

What is claimed is:
 1. An optical system used in a charged particle beam inspection system, comprising: one or more optical lenses; and a compensation lens configured to compensate a drift of a focal length of a combination of the one or more optical lenses from a first medium to a second medium.
 2. The optical system of claim 1, wherein the optical system is a position detection system comprising: a projection module including a projection lens and configured to project a first light beam to a sample; a receiving module including a receiving lens and configured to receive a second light beam reflected from the sample; and a detection module configured to detect a position of the sample based on the second light beam.
 3. The optical system of claim 2, wherein the one or more optical lenses include the projection lens included in the projection module and the receiving lens included in the receiving module.
 4. The optical system of claim 2, wherein the detection module is disposed in the first medium, and the projection module and the receiving module are disposed in the second medium.
 5. The optical system of claim 1, wherein the optical system is an imaging system comprising: an illumination module configured to project a first light beam to a sample; an objective lens configured to conjugate a second light beam reflected from the sample; and a detection module configured to detect an image of the sample based on the second light beam.
 6. The optical system of claim 5, wherein the one or more optical lenses include the objective lenses.
 7. The optical system of claim 5, wherein the detection module is disposed in the first medium, and the objective lens is disposed in the second medium.
 8. The optical system of claim 1, wherein the optical system is an illumination system comprising: an illumination module configured to project a first light beam to a portion of a sample under a charged particle beam; an objective lens configured to conjugate a second light beam reflected from the sample; and a detection module configured to detect an image of the portion of the sample based on the second light beam.
 9. The optical system of claim 8, wherein the one or more optical lenses include the objective lenses.
 10. The optical system of claim 8, wherein the detection module is disposed in the first medium, and the objective lens is disposed in the second medium.
 11. The optical system of claim 1, wherein the first medium is air, and the second medium is vacuum.
 12. The optical system of claim 1, wherein the charged particle beam inspection system is an electron beam inspection system.
 13. The optical system of claim 1, wherein a focal length of the compensation lens is configured based on a focal length of each one of the one or more lenses, a position of each one of the one or more lenses, and refractive indices of the first medium and the second medium.
 14. A method for operating an optical system used in a charged particle beam inspection system, the optical system including one or more optical lenses, the method comprising: providing a compensation lens in the optical system, the compensation lens being configured to compensate a drift of a focal length of a combination of the one or more lenses from a first medium to a second medium; assembling and calibrating the optical system in the first medium; removing the compensation lens; and placing the one or more optical lenses in the second medium.
 15. The method of claim 14, wherein the optical system is a position detection system, and the method further includes: projecting, by a projection module including a projection lens, a first light beam to a sample; receiving, by a receiving module including a receiving lens, a second light beam reflected from the sample; and detecting, by a detection module, a position of the sample based on the second light beam. 